Constraints on Neoproterozoic Intraplate magmatism in the Kalahari Craton: Geochronology and Paleomagnetism of ~890-795 Ma Extension-Related Igneous Rocks in SW Namibia and Adjacent Parts of South Africa

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Intraplate igneous rocks with ages of ~890-750 Ma are abundant along the margins of the Kalahari craton in southern Africa. They have the potential to provide useful piercing points for placing Kalahari within a Rodinia reconstruction, but in many cases are strongly tectonized within ~550 Ma orogenic belts. The best-preserved examples of these rocks, and probably the only ones suitable for paleomagnetic studies, occur near the SW craton margin in Namibia and adjacent parts of South Africa. We report results of ongoing, integrated geochronological and paleomagnetic studies of these units, including A-type felsic rocks of the Richtersveld suite (RS), which trend NE from the Neoproterozoic continental margin, and the younger, more extensive, N- to NE-trending Gannakouriep diabase dike swarm (GDS). The GDS is up to 300 km wide and records generation of large volumes of basaltic magma preceding final stages of rifting at ~750 Ma along the craton margin, which led to opening of the Adamastor Ocean.

We have obtained single-crystal U-Pb ID-TIMS zircon ages of ~890-880 Ma for granite and syenite within the largest RS plutonic complexes (Sjambok River and Older Bremen complexes) and for a NE-trending felsic swarm associated with the Older Bremen complex. Frimmel et al. (2001, Jour. Geol.) reported U-Pb zircon ages of ~830 and 800 Ma for two other igneous centers (Rooiberg and Klipbokkop) near the SW limit of the RS. New single-grain zircon and baddeleyite data from the largest dike in the GDS yield dates of ~795 Ma (this study; Rioux et al., 2010, Contrib. Mineral. Petrol.). These data indicate that intraplate felsic magmatism lasted for ~90 Ma and was followed closely by emplacement of the GDS.

An initial paleomagnetic data set from the RS and parts of the GDS yielded a single high-latitude pole (74o N, 234o E, n = 36, A95 = 4.5o) and included sites giving the dates mentioned above. The large differences in crystallization ages between these units suggest that the older sites were remagnetized during emplacement of the GDS. Ongoing work on additional GDS sites from other sampling areas yields distinctly different directions, suggesting that some of the dikes were emplaced at significantly different times; additional geochronological work is in progress to resolve this issue.

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Geological Society of America Abstracts with Programs, v. 43, issue 5, p. 371